As a method for obtaining a hollow foamed molded article, there has been hitherto developed a method, so called foam blow molding method, which includes melting and kneading a thermoplastic resin and a foaming agent in an extruder, extruding the obtained foamable molten resin through a die to obtain a foamed parison having a foamed layer, sandwiching the foamed parison in a softened state between molds, blowing a pressurized gas into the parison to mold the parison into a shape conforming to the inner shape of the molds.
The thus obtained hollow foamed molded article has generally a lighter weight and better thermal insulating property as compared with non-foamed hollow molded articles. Because of these properties, hollow foamed molded articles made of a polypropylene resin as a base resin are used as, for example, air conditioner ducts for automobiles.
Although polypropylene resins have excellent balance between their heat resistance property and mechanical strength, they are ill-suited for certain applications, especially applications in which they are used in a position near high temperature heat sources or applications in which they are used for refrigerators. In such applications, there is a demand for materials having high mechanical strength over a wider temperature range.
Polycarbonate resins, on the other hand, have higher heat resistance and better mechanical properties in a low temperature environment as compared with polypropylene resins and, additionally, exhibit self-extinguishing property.
As compared with general-use resins, such as polystyrenes, which are generally used in extrusion foaming, polycarbonate resins show a higher melt viscosity and a lower melt tension at a temperature suitable for foaming in the extrusion stage and, therefore, are inferior in foamability. In a molding method, such as foam blow molding, in which a foamed parison in a softened state is molded in a mold (such a method is hereinafter occasionally referred to as “foamed parison molding”), it is necessary that the foamed parison should maintain its softened state and keep its cell structure without causing excessive drawdown until it has been molded in the mold. Therefore, a resin used in the foamed parison molding is required to have a higher quality of foamability than that required in ordinary extrusion foaming.
Hitherto, foamed blow-molded articles in which a polycarbonate resin having foaming characteristics that are not good is used as a base resin have an expansion ratio of as low as about 1.3.
In this circumstance, Patent Document 1 proposes the use of a polycarbonate resin having a specific melt tension for the purpose of obtaining a foamed blow-molded article having a lower apparent density than conventional articles without causing cell breakage or drawdown of the foamed parison.
Patent Document 2 discloses a method in which a mixture of a crosslinking agent, such as a carboxylic anhydride, and a mixed resin containing a polycarbonate resin and polyethylene terephthalate is extruded and foamed. By so doing, a cross-linking structure is introduced into the polyethylene terephthalate dispersed in the polycarbonate resin, melt characteristics of the mixed resin, such as melt viscosity, so that melt tension and elastic property, are changed to a state suited to extrusion foaming, thereby enabling to produce an extruded foamed product.